Readout and fluxonioum ionization: lessons, pitfalls, and advice

The fluxonium qubit is a promising candidate to replace the transmon as the workhorse of cQED-based quantum processors. While remarkable recent progress has been made at the level of single- and two-qubit fluxonium gates [1-3], readout times and fidelities lag behind that of the transmon. To make matters worse, high-power readout of the transmon is plagued by the presence of multi-photon transitions, causing the transmon to ionize [4-6]. These transitions are also present in the fluxonium. One is then left to wonder if the fluxonium ionizes, and if so how to avoid it via smart design choices.



In this talk, we address these questions by systematically studying fluxonium ionization in a wide range of the fluxonium’s parameter space. We show that a careful optimization of the fluxonium’s inductance, capacitance, Josephson junction, and placement of the readout resonator’s frequency are all crucial to avoid detrimental multi-photon processes. Our work serves as an experimentally-relevant starting point when choosing one’s fluxonium-resonator parameters.

[1] L. Ding et al., Phys. Rev. X 13 (2023)

[2] H. Zhang et al., PRX Quantum 5 (2024)

[3] D. Rower et al., arXiv 2406.08295 (2024)

[4] D. Sank et al., PRL 117 (2016)

[5] M. Kherzi et al., Phys. Rev. Applied 20 (2023)

[6] M. F. Dumas et al., Phys Rev. X 14 (2024)